In the past, the scrap plastic and used plastic produced in the plastic processing process (hereinafter referred to as “waste plastic”) was incinerated or was disposed of in landfills. As a result, in the case of incineration, there were the problems that the high temperature of combustion damaged the incinerators and that reaction with chlorine caused the production of dioxins. Further, even with disposal in landfills, since plastic does not decompose and the soil does not solidify around it, there was the problem that the newly created land over the landfills was low in value of use.
As a means for dealing with this, various technologies are being put to use for recycling plastic. For example, plastic is being converted to oil or gas, however, this has the problem of a high treatment cost. On the other hand, dry distillation of plastic in a coke furnace is an economical method enabling a large amount of recycling. With dry distillation in a coke oven, coke can be recovered along with fuel gas and oily matter, so this is also an excellent method in terms of the diversity of possible applications.
The method of dry distillation of waste plastic in a coke oven is the method of mixing the waste plastic with coal, charging the mixture into the coke oven, then dry distilling it at about 1200° C. For example, this method is described in Japanese Patent Publication (A) No. 48-34901. While differing depending on the type of the plastic used, about 35% of the plastic becomes coke, about 25% oily matter, and about 40% coke oven gas. The coke derived from the plastic is discharged from the coke oven in a state mixed with the coke derived from the coal and is utilized as a reducing agent or fuel in a blast furnace or iron alloy making process etc.
As explained above, the method of dry distillation of waste plastic in a coke oven is an effective means for economical recycling of plastic. However, there has been no accurate knowledge regarding the relationship between the method of using plastic and the coke quality, so there have been problems in the quality of the coke produced. For example, in the means for recovering large amounts of gas or tar using the technology described in Japanese Patent Publication (A) No. 8-157834, there was no consideration given to coke quality and there was the problem that when a large amount of plastic was mixed in, the coke declined in strength. Note that since coke is used in blast furnaces, cupola furnaces, and other large sized facilities, it has to withstand the load conditions in these furnaces. High strength coke is therefore being sought. The deterioration of coke strength has become an important quality issue.
On the other hand, in the past, the readily available waste plastic produced in the processing of plastic (hereinafter referred to as “scrap plastic”) had been used in coke ovens. This scrap plastic was mainly chip-like in form, relatively high in purity, and of a shape able to be used in coke ovens as it was, so in the past there was no knowledge about the effects of its ash content on coke furnace operations or the effects of its apparent density. As a result, even when using used plastic produced from the home etc., which is poor in purity and mostly thin in shape (hereinafter referred to as “used plastic”), the same simple method was employed for using it in coke furnaces. When using this, however, there were detrimental effects on the quality of the coke.
Used plastic has problems due to its poor shape and low apparent density. If using large amounts of too small plastic, for example, plastic of a size of 5 mm or less or a thickness of 1 mm or less, there was the problem that the produced coke declined in strength. Further, if the plastic used was too large, the problem was observed of a drop in the yield of the lump coke. If using large amounts of poor grade, large ash content used plastic, there was the problem of a drop in the strength of the lump coke.
Japanese Patent Publication (A) No. 2001-49261 discloses the method of compression shaping waste plastic to obtain a plastic granular material with an apparent density of 0.40 to 0.95 kg/liter and mixing this with the coal by a mass ratio of 5% or less for dry distillation in a coke ovens. By using granulated plastic having an apparent density of 0.40 kg/liter or more, the problem of coke powdering can be solved. On the other hand, if trying to melt the plastic to shape it, problems arise such as the production of harmful gases at the time of melting, so the melting method is not considered an economical or safe method. As a condition for compression shaping plastic without melting it, the upper limit of the apparent density is deemed to be 0.95 kg/liter. The temperature of the plastic at the time of shaping is made 100° C. or more in order to remove the moisture content of the plastic. If the temperature exceeds 160° C., part of the plastic will start to melt and harmful gases will be produced, so 160° C. or less is considered preferable.
Further, in the method of treatment of waste plastic in a coke furnace described in Japanese Patent Publication (A) No. 2002-12876, the method is disclosed of dechlorinating the waste plastic at a temperature of 300° C., compression shaping it to adjust the density to 0.78 to 1.0 g/cm3, and mixing a predetermined amount of the shaped waste plastic into a coke oven for dry distillation.